Variable condenser



May 16, 1961 1..r. BARNES VARIABLE CONDENSER 4 Sheets-Sheet 2 Filed March 20, 1958 INVENTOR.

LLEWELL Y/V 7. BARNES May 16, 1961 T. BARNES 2,984,776

VARIABLE CONDENSER Fi led March 20A 1958 4 Sheets-Sheet 3 O FIG. 8.

INVENT OR.

LL EWEL LYN 7T BARNES WWW A TTORNIYS L. T. BARNES VARIABLE CONDENSER May 16, 1961 4 Sheets-Sheet 4 Filed March 20, 1958 FIG. l7.

FIG. l6. FIG. \5.

ATTORNEYS United States Patent VAiRIABLE CONDENSER Llewellyn T. Barnes, 155 Aflanfic Ave, Freeport, NY, assignor of thirty-three and one-third percent to Charles T. Barnes, Freeport, and thirty-three and onethird percent to John F. Woog, Garden City, N.Y.

Filed Mar. 20, 1958, Ser. No. 722,773

9 Claims. (Cl. 317-249) My invention relates to improvements in trimmer condensers.

In particular, my invention relates to improvements in trimmer condensers of the type disclosed in my prior U.S. Patent No. 2,607,826, issued August 19, 1952, for Trimmer Capacitor.

In accordance with my invention, I provide a condenser having an elongated hollow casing which has a high dielectric constant, said casing having at least one fixed electrode fixed to the outside thereof. The movable electrode is slidably positioned within the casing and can be moved by means of an axially extending adjusting device which includes a screw.

One important object of my invention is to provide improved adjusting devices for said movable electrode, such as to minimize back-lash and such as to ensure the highest possible approach to linear relationship, between axial movement of the movable electrode and angular movement of the adjusting screw.

Another important object of my invention is to provide improved adjusting devices such as to reduce the length of the trimmer condenser.

Another important object of my invention is to provide a design of the fixed electrodes and of the casing which will ensure the highest possible linear relationship between axial movement of the movable electrode and electrical capacity between the electrodes. In particular, my invention makes it possible to more closely approach to zero capacity between the electrodes than has heretofore been possible.

Other objects and advantages of my invention will become apparent from the following description in conjunction with the annexed drawing, in which preferred embodiments are disclosed.

In the drawings,

Figs. l-S of the drawing show a first embodiment of the invention.

Fig. 1 is a longitudinal section of said first embodiment.

Fig. 2 is an exploded longitudinal section of the elongated split nut through which the adjusting screw extends, taken on line 2-2 of Fig. 3.

Fig. 3 is an exploded cross-section of said split nut.

Fig. 4 is an exploded elevation of said split nut assembled with a ring, and of the head which is keyed to the adjusting screw.

Fig. 5 is a section on line 5-5 of Fig. 1.

Figs. 6-7 show a second embodiment, Fig. 6 being a longitudinal section thereof.

Fig. 7 is a section on line 7-7 of Fig. 6.

Figs. 8-10 show a third embodiment, Fig. 8 being a longitudinal section thereof.

Fig. 9 is a section on line 9-9 of Fig. 8.

Fig. 10 is a section on line 10-10 of Fig. 8.

Figs. 11-16 show a fourth embodiment, Fig. 11 being a longitudinal section thereof.

Fig. 12 is a section on line '12-12 of Fig. 11.

Fig. 13 is an end view of one of the components of the fourth embodiment.

Fig. 14 is a section on line 14-14 of Fig. 13.

Fig. 15 is an end view of another component of the fourth embodiment.

Fig. 16 is a side elevation of the component of Fig. 15.

Fig. 17 is an explanatory diagram in connection with certain features of the invention.

"ice

First embodiment (Figs. l-S) For convenience, referring to Fig. 1, the left-hand end of condenser 10 and its parts is referred to as the front end, while the right-hand end is referred to as the rear end.

Figs. 1-5 show a condenser 10 which has a generally cylindrical base 11 whose peripheral wall 13 is of generally uniform external diameter and has a bore 12. Near its front end, said peripheral wall 13 is provided with a transverse, peripherally extending, annular flange 14. Forwardly of flange 14, said peripheral wall of base 11 has a smooth exterior. The front end portion 12a of bore '12 is screw-threaded and is of increased diameter so as to define a transverse annular shoulder 12b at the rear end of bore portion 12a. Rearwardly of flange 14, wall 13 of base 11 has an external screw thread 15.

Clamping nut 16 is adapted to be screwed onto threading 15, and wall 13 may be extended through an opening in a support (not shown) with said support being clamped between flange 14 and nut 16.

A hollow, elongated cylinder 17 is mounted upon wall 13 of base 11, forwardly of flange 14 and in endwise abutment therewith. Said wall 13 extends frictionally into and is afiixed to the rear end portion of cylinder 17, with said cylinder 17 being optionally secured to flange 14 by means of solder 19.

Said cylinder 17 is made of a material having a high dielectric constant, which material may optionally be glass, quartz, polystyrene, or ceramic alumina.

The fixed electrode of condenser 10 is in the form of a band 18 of silver or other conductive metal, which is applied in the form of a coating on the outer surface of cylinder 17 near the front end thereof. Optionally, said band 18 may be almost cylindrical and may almost completely encircle cylinder 17. Optionally, a split metal ring 9 which bears a terminal post or lug 8 may be snapped over band 18, in the manner clearly shown in Figs. 1 and 5, and soldered at 9a to silver band 18.

Optionally, the front end portion of cylinder 17 is externally bevelled at 20 and internally bevelled at 20a, in the manner shown in Fig. 1. Optionally and preferably, a cylindrical end plug 36 having an enlarged head 36a is force-fitted into the front end portion of cylinder 17 with head 36a abutting the front end of cylinder 17. Preferably, electrode 18 is located rearwardly of and axially spaced from the rear face of plug 36.

Base 11 also includes a front end member 28 of cylindrical shape. This member 28 has an external screw thread 28a and is screwed into the threaded portion 12a of bore 12, with the rear end of member 28 abutting shoulder 12b. End member 28 extends forwardly of wall 13 and has a front end wall 28b. Said end member 28 has a bore 7 of the same diameter as the main portion of bore 12. The rear end portion 7a of bore 7 is conically tapered and meets with the inner face 280 of end wall 28b. As will be described in detail below, said bore portion 7a serves as a seat.

Pin or lug 34 extends radially with respect to base wall 13 and is embedded therein and extends into a radial opening within end member 28, as is clearly shown in Fig. 1, so as to prevent end member 28 from turning relative to wall 13 in the final assembly. This pin 34 is driven through wall 13 and the wall of end member 28 prior to assembly of these parts with cylinder 17.

The movable electrode is in the form of a hollow cylindrical piston 23 axially slidably located within cylinder 17. This piston 23, as well as base 11, and all other metal parts of the device, are preferably made of conductive metal which has a low coefiicient of thermoexpansion, which is preferably substantially the same as the coefiicient of thermo-expansion of cylinder 17. For example, said metal parts may be made of Invar, an iron-nickel alloy.

Said piston or cylinder 23 is hollow, but has a transverse wall 23a somewhat near the middle thereof. Said piston 23 is shown in its frontmost position corresponding to maximum capacity, in Fig. 1. As also shown in Fig. 1, the peripheral wall of piston 23 and the location of wall 23a are such that piston 23 may be moved to a rearmost position, corresponding to minimum capacity, wherein wall 23 extends loosely around the periphery of member 28 with the rear face of wall 23a located'close to the front face of said end member 28.

In order to move piston 23, a bar 21 is provided which is optionally square in cross-section as shown in Fig. 5. The front end of said bar 21 extends through a corresponding bore 23b in wall 23m. A stop member 21a, which may optionally be a retaining ring is fixed to bar 21 at the rear face of wall 23a.

The front end portion of bar 21 is of reduced diameter and optional cylindrical shape and extends through a corresponding square bore 23b in wall 23a, with bar shoulder 21 abutting wall 23a. The extreme front end portion 21b of bar 21 is screw-threaded, and washer 21c and lock nut 21d are secured to said bar portion 21b in the usual manner, forwardly of wall 23a, so as to prevent axial movement of bar 21 relative to wall 23.

The other end of bar 21 extends slidably through a square opening 28d in wall 281). The rear end of bar 21 is fixed by any suitable means (not shown) to a short screw 25.

The movement of screw 25 is controlled by an axially extending elongated split nut of considerably greater length having elements 26 and 27, as well as by ring 5, spring 6 and key 30.

Said split nut 26, 27 is shown in detail in Figs. 2 and 3. Nut element 26 corresponds generally to the major portion of a cylinder and element 27 corresponds generally to the missing minor portion of a cylinder, so that the two elements 26 and 27 together have a generally cylindrical shape. In cross section, as shown in Fig. 2, the missing segment of element 26 is wedge-shaped, to define a longitudinal slot 1, and element 27 is correspondingly wedge-shaped. The elements 26 and 27 have respective V-shaped screw threads 26a and 27a. The threads 26a and 27a must be held in alined relationship to define the complete internal thread of the nut 26, 27, and this is done by the interfitting relationship of screw 25, elements 26 and 27, base 12, ring 5, spring 6 and key 30, the complete interfitting relationship of these parts being shown in Figs. 1 and 5. In the interfitted relationship of said parts, screw 25 is threaded into the composite screw threads 26a, 27a.

The major split nut element 26 has a conically tapered front end surface 261) which is seated upon seat surface 7a of end wall 28b. The front end of the minor split nut element 27 is spaced from said seat 7a. The front portions of outer peripheral surfaces of the split members 26 and 27 together fit frictionally turnably within the composite bores 12 and 7. The rear portions 26c and 270 of elements 26 and 27 are of reduced diameter so as to provide clearance for cylindrical ring 5. Ring is preferably soldered at 5a to the rear end of the main portion of element 27 and is rearwardly spaced from the rear end of the main portion of element 26. Elements 26c and 270 together extend turnably through the bore of ring 5. The reduced portion 260 of element 26 extends slightly rearwardly of the reduced portion 27c of element 27, as is clearly shown in Fig. 1.

Key 30 has a sonically bevelled rear surface 301: which normally bears against the corresponding conical seat 7b in the rear end of bore 7, as is clearly shown in Fig. 1. Key member 30 has a pair of forwardly extending, diametrically opposed projections 30b and 300. Projection 300 is alined with and shaped like element 27c and extends slidably into slot 1. Projection 35b extends slidably into a short slot 1a in element 26 which is diametrically opposite slot 1. Coil spring 6 extends around split nut 26, 27 and projections 30b and 30c and its ends bear respectively against ring 5 and key 36, urging ring 5 and key 30 apart.

Normally, as shown in Fig. l, the front face of shank 30b is spaced from the front end of slot 1a. The conical face 7b of key member 30 has radially extending grooves 302, clearly shown in Figs. 1 and 4. A pin 31 is fixed in member 11, at right angles to seat 7b, and protrudes slightly past seat 7b into bore 7. Normally, said pin 31 extends into one of the grooves Stle and prevents key member 30 from turning. Key member 30 has the usual slot 30 in the rear face thereof, for reception of a screw-driver or the like.

In order to operate the device, a screw-driver may be placed in slot 36 and key member 30 pressed forwardly until pin 31 is clear of groove 302. Key member 30 may then be turned, thereby turning the split nut member 26. This causes split nut member 27 to turn in unison with member 26, and thereby moves screw 25 axially. Accordingly, the movable electrode 23 is moved forwardly or rearwardly depending upon the direction of turning of key member 30, and may readily be. locked with pin 31 again extending into one of the grooves 30e. This makes it possible to move screw 25 through a fixed selected linear distance, making it possible to change the capacity of the unit in fixed selected increments.

In the condition of Fig. 1, key member 30 rigidly prevents split nut member 26 from turning. As the result of the action of spring 6 upon ring 5 and hence upon split nut 27, a longitudinal or axial tension is exerted by the threads of key member 27 upon the threads of screw 25, relative to the threads of member 26. Also, members 26 and 27 are held under outward radial tension against the wall of bore 7. As a result, it is virtually impossible for there to be any movement of screw 25. Since screw 25 is continuously under tension between the elements 26 and 27, upon turning element 26 by means of key member 30, screw 25 is immediately moved axially without any lost motion.

From the foregoing, it will be seen that the movement of electrode 23 is mechanically in complete linearity with the turning angular movement of key member 30.

Optionally, the rear end of the unit may be sealed by means of gasket or packing 3 and cap nut 35, which is screwed onto the rear end portion of thread 15, rearwardly of nut 16.

Second embodiment (Figs. 6-7) This embodiment is similar in principle to the first embodiment, the chief difference being that in the second embodiment the condenser body 57 is elliptical in shape rather than cylindrical in shape, as will readily be apparent from a comparison of Figs. 5 and 7.

Body 51 corresponds quite closely to body 11 of the first embodiment, except that the front portion of body 51, forwardly of flange 54, is elliptical in shape in order that it may be fitted into the bore of body 57. Body 57 is optionally soldered at 54a to flange 54. Also, in the second embodiment, end closure portion 68 of body 54 is optionally integral therewith.

Fixed electrode 58 and lug 49 correspond generally to the corresponding elements 18 and 9 of the first embodiment. Optionally, the movable electrode 63 of the second embodiment is somewhat modified from the electrode 23 of the first embodiment. Said movable electrode 63 optionally has a rear element 100 of outer ellipticalcrosssectional shape, having a front end wall 101.v Shaft 61 is optionally cylindrical in shape and extends to the rear face of wall 101 and has an extension 61a of reduced diameter which extends through a corresponding opening in wall 101.

Said movable electrode 63 has a front element 102 of elliptical outer cross-sectional shape which butts against the front face of wall 101. Said element 102 has a front recess 102a. Shaft 61 has a rear portion 61b of slightly smaller diameter than portion 610, which extends turnably through an opening in member 102, and which has a threaded portion 610 within recess 102a. Washer 103 and nut 104 are secured over the threaded shaft portion 610 in the usual way.

In order to provide tension upon wall 101 and element 102, element 102 is provided with an angular recess 102b extending from its rear wall. Coil spring 105 is located in said recess 102b. One end 105a of spring 105 is located in said recess 10211. One end 105a of spring 105 is turned so as to extend longitudinally, and extends into a selected one of a series of longitudinal holes 106 in end wall 101. The hole 106 is selected, depending upon the desired tension which is to be exerted by spring 105 upon elements 63 and 102. Spring 105 exerts tension upon element 102 against washer 103 and also in the circumferential direction, and thereby ensures against any turning of the movable electrode within the bore of body 57. This makes it unnecessary to provide an exact fit of the movable electrode within the bore of body 57.

Shaft 61 extends turnably through an opening in closure portion 63. The rear end of shaft 61 is integral with screw 25, and this screw 25 and split nut 26, 27, ring 5 and spring 6 are substantially the same as in the first embodiment.

Key 60 is similar to key 30 of the first embodiment, except for the optional elimination of pin 31 and grooves 30e. In addition, instead of slot 30 key 60 is optionally provided with an axially extending driving shank 110 which extends rearwardly thereof. Instead of end cap 35, cap 65 is provided with a threaded shank 65a of reduced diameter which may be threaded into a rear end recess 111 of body 51. Said cap 65 is prevented from turning by set screw 1 12, which also serves to hold a pointer 113 in place.

Said cap 65 has a rear portion 65b of reduced diameter which is hollow and which is internally threaded to receive an externally threaded closure nut 114. The recess of element 65b contains packing 115 and gland 115a, forwardly of nut 114, and shank 110 extends turnably through appropriate recesses in cap 65 and nut 114 to the rear of said nut 114. A turning head 1 16 is fixed upon the rear end of shaft 110 by means of set screw 117. This head 116 may optionally be calibrated in any suitable manner to be read by means of a pointer 113. Accordingly, by turning head 116 and hence key member 60, it is possible to adjust the condenser entirely similarly to the first embodiment.

Optionally, as in the first embodiment, a plug may be inserted in the front of the bore of condenser body 57 in order to seal same.

The elliptical shape of the movable electrode 63 and fixed electrode 58 of the second embodiment is highly advantageous. Reference is made to the diagram of Fig. 17. Fig. 17 shows a hollow charged body 129 of elliptical cross-sectional shape. The broken line 130 is a graph of the charge distribution on the outer surface of body 129 and shows a greater charge potential at the more highly curved parts of body 129. For a cylindrical charged body, the graph of charge distribution would be a circle.

The eliptical shape of the electrodes of the condenser is possibly not of great importance when the movable electrode is in its position of the drawings, such as Figs. 5 and 6, corresponding to maximum capacity. However, this elliptical shape of the electrodes becomes important when the movable electrode is moved rearwardly toward its position of minimum capacity. When the movable electrode has almost approached its position of minimum capacity, it is located wholly to the rear of the movable electrode. In that case, the two electrodes are virtually in end-wise opposition to each other. Accordingly, at that point, the condenser is for practical purposes a condenser consisting of two plates (the end surface of the electrodes) in opposition to each other. These end surfaces correspond to plates or disks, because most of the charge of the flat plates of a condenser is disposed around the periphery of said plates.

It is possible that theoretically the capacity of a condenser with circular or round plates should be approximately the same as the capacity of a condenser with elliptical plates. However, in a condenser having elliptical plates, the charge on the plates is concentrated at the points of maximum curvature, or minimum radius of curvature. Accordingly, in practice, as the elliptical plates are moved further apart, the actual capacity drops more slowly and in a more linear manner than is the case if the plates are round or circular.

Since the elliptical electrodes in accordance with my invention function very much like plate electrodes as the device approaches minimum capacity, the elliptical shape of the electrodes makes it possible to approach zero readings in a more linear manner, corresponding to axial movement of the movable electrode, than would be possible if the electrodes were cylindrical. This improvement makes it unnecessary to vary the contour of the fixed electrode in the manner disclosed in my prior US. Patent No. 2,607,826. In fact, I have found that it is possible with my improved condenser to obtain a linear response which extends through zero capacity to a point in which the device functions as an inductance, as the movable electrode approaches its extreme rearward position.

Third embodiment (Figs. 8-10) The overall length of the condenser of the third embodiment is shorter than the length of the condensers of the first two embodiments by reason of the fact that a non-conducting screw is employed. Figs. 8-10 show a condenser body 70, which is elliptical in cross-section, and a base 71 of elliptical outer cross-section. This base 71 has a flange 72 which abuts the rear end of condenser body 70, and which is optionally secured thereto by solder 73. Forwardly of flange 712, base 71 has a smooth peripheral surface and is fitted into body 70. Rearwardly of flange 72, base 71 is screw-threaded at 71a to receive the clamping nut 91. Said base 71 has a cylindrical through bore 74 which is of reduced diameter and screwthreaded at its rear end portion 74b. The fixed electrode 75 and lug 76 secured thereto by solder 76a require no detailed description, since they are similar to the corresponding elements in the first two embodiments.

The movable electrode 77 is elliptical in external crosssectional shape and is slidably positioned within body 70. Said movable electrode 77 optionally has a front end recess 77a and a rear end recess 77b and has an annular cylindrical bore 770 extending from the front face of recess 77b part-way toward recess 77a.

Said movable electrode 77 also has a central, axially extending, screw-threaded through bore 77d. Instead of the metal split nut of the first two embodiments, in this third embodiment I employ a split screw which is made of non-conducting material, which is optionally glass of very great strength and having a low co-efiicient of expansion. Said screw comprises the split elements 78 and 79. As shown in Figs. 8 and 9, said screw 78 corresponds to the major portion of a cylinder, and element 79 corresponds to a wedge of pie-shaped segment which completes the cylinder. Said elements 78 and 79 are respectively externally screw-threaded at 78a and 79a to 7 make a composite screw thread for turning engagement within threaded bore 77b.

Rearwardly of the threads 78!: and 7%, the elements 78 and 79 have smooth peripheries which together extend externally through bore 74a. Rearwardly of bore portion 74a, element 73 has an outwardly extending peripheral flange 84 which abuts the shoulder 81 formed at the junction between bore 74a and bore 74. The corresponding flange 82 of element 79 is rearwardly spaced from said shoulder 81. Rearwardly of flanges 81 and 82, ring 83 is turnably mounted upon split screw elements 7 8 and 79 and abuts flange 82 and is rearwardly spaced from flange 811 Element 78 extends rearwardly of element 79 and is formed with a lateral cross-slot 84. Key head 85 has a rectangular shank 86 which extends axially slidably into slot 84, in the manner'clearly shown in Figs. 9 and 10.

Said head '85 is bevelled at 8511. Closure nut 87 is screwed into bore 745 and is bevelled at 87a to provide a seat for the bevelled portion 35:: of head 85. Said head 85 has a lateral slot 85]) for reception of a screw-driver or the like through the central opening of closure nut 87. Coil spring 83 extends around the split screw elements 78 and 79 and extends between ring 83 and key head 85.

A weak metal contact spring 90 extends around the threaded portions of elements 78 and 79. One end portion of spring 911 is seated within recess 77c and bears against the front face thereof. The other end portion of spring 90 is seated within an annular recess 91 in the rear face of base member 71. The sole purpose of spring 90 is to provide electric contact between movable electrode 77 and base 71, so that the movable electrode may be connected in circuit via base 71, which may be grounded.

It will be apparent that the principle of the split screw of the third embodiment is similar to the principle of the split nut of the first two embodiments. Tension is exerted by spring 88 and ring 83 upon split screw element 79, thereby exerting tension upon the threads of bore 77b and preventing slippage. On the other hand, to turn the screw, tension is exerted via key head 85 upon the other screw element 78 alone, and is thereby indirectly transmitted to the element 79.

Fourth embodiment (Figs. 11-16) This embodiment is also extremely compact and also can be readily assembled.

The drawing of this embodiment shows a condenser body 131 of elliptical shape and having fixed electrode bands 132 and 133 mounted upon the outer periphery thereof. These bands 132 and 133 optionally extend around the entire periphery of body 131 and are axially spaced. Lugs 134 and 135 are respectively fixed to said electrodes 132. and 133.

The movable electrode 136 of this embodiment includes a pair of axially spaced sleeves 1 37. Each of these sleeves 137 has an elliptical outer shape roughly conformingto the shape of the bore of body 131, and has a cylindrical internal bore 137a. Sleeve 137 also has diametrically opposed ears 13712 extending into bore 137a and each extending around approximately one fourth of the periphery of bore 137a. The ears 2137b of the respective sleeves 137 extend from the proximate ends thereof to points slightly short of the remote ends thereof, as shown in Fig. 11.

Movable electrode 136 also includes a generally cylindrical element 138. This element 138 extends slidably through bore 137a. Element 138 is provided at its respective ends with diametrically opposed flange segments 133a which extend outwardly therefrom and which are located in the bore portions 137a, in the manner clearly shown in Figs. 11 and 12. Each of these segments 138a extends slightly less than one-fourth the circumference of element 138.

In assembled position, each flange segment 138a abuts the end of an car 1371). By turning element 138 one quarter of a revolution relative to elements 137, the elements may be disassembled. A coil spring 142 extends around element 138 intermediate the elements 137, with the end 142a of spring 142 extending into respective holes 143 in the approximate faces of elements 137. Spring 142 thereby maintains the assembly of the two elements 137 and element 133 under longitudinal tension relative to each other and under torsional tension within the bore of body 131.

Said element 138 has a hollow bore 139 whose central portion 139a is of reduced diameter and is threaded. The major split nut element 149 and minor split nut element 141 are entirely similar to the corresponding split nut elements of the first embodiment and are threaded into bore portion 137a in the manner shown in Fig. 11.

A pin 144- extends through registering holes in one of the elements 137 and element 138, so as to prevent turning of element 133 relative to the elements 137. As a result, the turning of the split nut element 140 and 141 causes an advance or retraction as the case may be, of movable electrode 136.

The front end portion of the assembly of Fig. 11 is not shown in detail, since it may be similar to preceding embodiments such as the first embodiment.

While I have disclosed preferred embodiments of the invention, and have indicated various changes, omissions and additions which may be made therein, it will be apparent that various other changes, omissions and additions may be made in the invention, without departing from the scope and spirit thereof.

I claim:

1. In a variable electrical reactance unit having a hollow longitudinal body and an adjustable reactance varying member longitudinally slidably positioned within said body, a drive assembly for said reactance varying member, said drive assembly comprising an elongated propulsion member which extends longitudinally and which is turnably disposed within said body in longitudinal alinement with said reactance varying member, said propulsion member being divided by longitudinal cuts into a major element and a minor element, said major element having the cross-sectional shape of the major sector of a circle and said minor element having the cross-sectional shape of the minor sector of a circle, said body having means exerting longitudinal tension upon one of said propulsion member elements, said unit having means external of said body for turning one of said propulsion member elements, and means coupling said propulsion member elements to said reactance varying member so as to move same longitudinally upon turning said propulsion member.

2. A unit according to claim 1, said propulsion member having a composite screw-threaded longitudinal bore, said coupling means including a screw disposed within said propulsion member here and means connecting said screw and said reactance varying member.

3. A unit according to claim 1, said propulsion member being non-conducting, said reactance varying member having a screw-threaded longitudinal bore, said propulsion member being threaded through said bore of said reactance varying member.

4. A trimmer capacitor comprising an insulator body of generally elliptical cross-sectional shape having a longitudinal bore, a conductive coating on the outside of said body, a conductive element of generally elliptical cross-sectional shape longitudinally slidably disposed within said bore, said element being in two longitudinally successive portions and including means torsionally tensioning said portions, and means for sliding said conductive element.

5. A trimmer capacitor comprising an insulator body of generally elliptical cross-sectional shape having a longitudinal bore, a conductive coating on the outside of said body, a conductive element of generally elliptical cross-sectional shape longitudinally slidably disposed within said bore, said conductive element comprising longitudinally spaced hollow end pieces, a cylindrical body extending through said end pieces, means releasably and loosely interlocking said body to said end pieces, means tensioning said end pieces both torsionally and also longitudinally against said end pieces, and means for sliding said conductive element.

6. In a variable electrical reactance unit having a hollow longitudinal body and an adjustable reactance varying member longitudinally slidably positioned within said body, a drive assembly for said reactance varying member, said drive assembly comprising an elongated propulsion member which extends longitudinally and which is turnably disposed within said body in longitudinal alignment with said reactance varying member, said propulsion member being divided by longitudinal cuts into a major element and a minor element, said body having means exerting longitudinal tension upon one of said propulsion member elements, means accessible externally of said body and extending into said body and coupled to said propulsion member for turning same, and means coupling said propulsion member to said reactance varying member so as to move same longitudinally upon turning said propulsion member.

7. A combination in accordance with claim 6, said propulsion member being disposed wholly within said body, said reactance varying member and said drive assembly being made of the same material.

8. A trimmer capacitor comprising a hollow longitudinally extending insulator body, a conductive coating on the outside of said body spaced from one end thereof, a conductive element longitudinally slidably disposed within said body and movable between a position proximate to said body end and a position of opposition to said conductive coating, a drive assembly for said conductive element comprising an elongated propulsion member which extends longitudinally and which is turnably disposed within said body adjacent said body end in longitudinal alignment with said conductive element, said propulsion member being divided by longitudinal cuts into a major element and a minor element, said body having means exerting longitudinal tension upon one of said propulsion member elements, means accessible externally of said body end and extending through said body end into said body and coupled to said propulsion member for turning same, and means coupling said propulsion member to said conductive element so as to move same longitudinally upon turning said propulsion member.

9. A trimmer capacitor in accordance with claim 8, said propulsion member being disposed wholly within said body, said conductive element and said drive assembly being made of the same metal.

References Cited in the file of this patent UNITED STATES PATENTS 1,548,801 Jacobs Aug. 4, 1925 2,350,823 Robinson June 6, 1944 2,754,458 Shull July 10, 1956 FOREIGN PATENTS 523,584 Great Britain July 17, 1940 

